1. Field of the Invention
The present invention relates to a nanotube cartridge that has nanotubes protruding from a tip edge thereof and further to a method for manufacturing such a nanotube cartridge.
2. Prior Art
In recent years, scanning probe microscopes (SPM) such as tunnel microscopes (STM), atomic force microscopes (AFM), etc. have been developed. These devices detect information concerning the physical properties of sample surfaces at the atomic level. When obtaining information concerning the physical properties of sample surfaces using scanning probe microscopes, a probe needle that detects information by directly contacting the sample surface is required.
Such probe needles are currently constructed from a semiconductor cantilever. In this semiconductor cantilever, a protruding portion is formed on a cantilever portion, and the tip end of this protruding portion is worked to a sharp point. The sharpened tip end of the protruding portion forms a probe needle point. Information on physical properties such as information concerning atomic structure, magnetic information, information concerning functional groups, information concerning electrons, etc. are obtained by causing this tip end to contact the sample surface so that physical and chemical interactions with the sample surface are detected.
The resolution of information concerning physical properties naturally increases when the probe needle point has an increased sharpness. However, even if the tip end of the protruding portion is sharpened using semiconductor techniques, it is difficult at the current technical level to reduce the diameter of the tip end to a value that is less than several tens of nanometers. Under these circumstances, carbon nanotubes were discovered, and a carbon nanotube probe needle in which a carbon nanotube is adhered to the protruding portion was proposed by H. Dai et al. in NATURE (Vol. 384, Nov. 14, 1996).
The diameters (D) of carbon nanotubes range from approximately one nanometer to several tens of nanometers, and such carbon nanotubes have an axial length (L) of up to several microns. The aspect ratio (L/D) of such carbon nanotubes ranges from several hundred to several thousand, and these carbon nanotubes have optimal properties for use as probe needles in scanning probe microscopes. However, in conventional carbon nanotube probe needles, carbon nanotubes are simply caused to adhere to the protruding portion. As a result, such probe needles have drawbacks in that a few scans of the sample surface with the carbon nanotube causes the carbon nanotube to fall off of the protruding portion, and the probe needle effect is lost.
Accordingly, the inventors of the present application invented two methods for firmly fixing a carbon nanotube to the protruding portion of cantilever portion. In one method, the carbon nanotube is covered and fastened to the surface of the protruding portion by a coating film. This method is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2000-227435. In another method, the base end portion of the carbon nanotube is fused to the surface of the protruding portion by subjecting this base end portion to electron beam irradiation or by passing an electric current through this base end portion. This method is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2000-249712.
In these Japanese Laid-Opened applications, the inventions are not limited to carbon nanotubes. The inventions are applicable to nanotube probe needles that use nanotubes in general, such as BN type nanotubes (boron nitride), BCN type nanotubes (boron carbonitride), etc.
In the above Japanese Laid-Opened (Kokai) applications, a nanotube cartridge in which carbon nanotubes are lined so as to protrude on a knife edge is used when the carbon nanotube probe needle is manufactured.
Conventional nanotube cartridge manufacturing methods will be described below.
First, carbon nanotubes (CNT) are purified by electrophoretic method disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2000-72422. CNTs are purified by dispersing a carbon mixture in an electrophoretic solution, and applying a direct-current voltage or alternating-current voltage. When a direct-current voltage is applied, the CNTs are lined up in a straight row on the cathode. When an alternating-current voltage is applied, CNTs are lined up in straight rows on both the cathode and anode as a result of the formation of a non-uniform electric field. This electrophoretic method is usable so as to purify not only carbon nanotubes but also BCN type nanotubes and BN type nanotubes. In the description below, these nanotubes, including carbon nanotubes, will be collectively referred to “nanotubes”.
Next, a process in which the purified nanotubes are dispersed in dispersion solution and then caused to adhere to a knife edge by electrophoresis will be described.
FIG. 11 shows the manner of manufacturing a nanotube cartridge that uses direct-current electrophoresis.
An electrophoretic solution 20 that contains dispersed nanotubes is placed inside a hole in a glass substrate 21. Knife edges 22 and 23 are disposed so as to face each other in this solution, and a direct-current power supply 18 is applied. The knife edges 22 and 23 have sharp blade tips 22a and 23a at their tip ends. Countless, extremely small nanotubes that are invisible to the naked eye are present in the electrophoretic solution 20. The nanotubes are charged as a result of contact with the electrophoretic solution and are caused to move by the electric field. When the electrophoretic solution is isopropyl alcohol, the nanotubes are caused to adhere to the blade tip 22a of the knife edge 22 of the cathode in a perpendicular state. This arrangement of the nanotubes can be confirmed by an electron microscope.
FIG. 12 shows the manner of manufacturing a nanotube cartridge that uses alternating-current electrophoresis.
The manner of nanotube cartridge manufacturing in FIG. 12 is similar to that shown in FIG. 11. Only difference is that an alternating-current power supply 19 is applied via an amplifier 26. A non-uniform electric field acts between the two cathodes. In long slender objects such as nanotubes, the polarized charge induced in the nanotubes senses the non-uniform electric field so that the nanotubes undergo electrophoresis. In this case, amorphous particles are immovable. Accordingly, unlike the direct-current method, the alternating-current method has a particle discriminating function. Even if a non-uniform electric field is not deliberately constructed, local non-uniform electric fields are formed in actuality. Thus, electrophoresis can be realized. In the alternating-current method, an alternating current of, for instance, 5 MHz, 90 V is applied. Nanotubes adhere perpendicularly to the blade tips 22a and 23a of the knife edges of both electrodes.
FIG. 13 shows a completed nanotube cartridge.
The nanotube cartridge A is comprised of the knife edge 23 and nanotubes 4 that adhere to the blade tip 23a of this knife edge 23 in a substantially perpendicular fashion relative to the knife edge 23. The nanotubes 4 are perpendicular to the blade tip 23a, and some of them are at an oblique angle. In some cases, furthermore, nanotubes 4 are gathered together and adhered in bundles.
FIG. 14 shows a manner that transfers nanotubes to an AFM cantilever.
A cantilever 27 is a silicon element that comprises a cantilever portion 28 and a protruding portion 29 that is formed at the tip end of the cantilever portion 28. A nanotube cartridge A is disposed so that the nanotubes 4 face the protruding portion 29. The cantilever is movable and adjustable three-dimensionally (XYZ), and the nanotube cartridge A is movable two-dimensionally (XY). By way of these movements and adjustments, a nanotube 4 is transferred to protruding portion 29 so that the tip ends 4c of the nanotube adheres to the protruding portion 29. These operations are performed while being magnified and projected in an electron microscope compartment 30.
FIG. 15 shows a completed nanotube probe needle B.
The base end portion 4b of the nanotube 4 is fastened to the protruding portion 29 by a coating film and/or fusion. The above-described tip end region 4c now becomes the base end portion 4b. The tip end portion 4a of the nanotube 4 acts as a probe needle. The tip end portion 4a contacts with a sample surface, so that information on the physical properties of the sample is detected by, for instance, an AFM operation.
It has been found that, by using of such a nanotube probe needle as completed as described above, a high-resolution image of the sample surface can be detected as a result of the superior aspect ratio of the nanotube. Furthermore, as a result progress made in recent research, numerous shining results have been obtained in various fields such as physics, chemistry and biochemistry, etc.
While such a nanotube probe needle B has superior properties, a nanotube cartridge A must be used to manufacture this nanotube probe needle B. However, since this nanotube cartridge A is manufactured by the electrophoretic method, not only is a high degree of technology required, but complicated processes using numerous parts are required. Thus, the manufacturing cost is high, creating a factor that hinders the popularization of such nanotube probe needles B.